A buffer amplifier (a.k.a. buffer) is an electronic device that provides electrical impedance transformation from one circuit to another. Two main types of buffers exist: the voltage buffer and the current buffer. Typically a current buffer amplifier is used to transfer a current from a first circuit, having a low output impedance level, to a second circuit with a high input impedance level. The interposed buffer amplifier inhibits the second circuit from loading the first circuit unacceptably and interfering with its desired operation.
In the ideal current buffer the input resistance is zero while the output resistance is infinite (impedance of an ideal current source is infinite). Other properties of the ideal buffer typically include perfect linearity regardless of signal amplitudes and instant output response regardless of the speed of the input signal. For a current buffer amplifier, if the current is transferred unchanged (the current gain is 1), the amplifier is called a unity gain buffer or a current follower because the output current “follows” or tracks the input current. The current gain of a current buffer amplifier is (approximately) unity. Existing current buffer amplifiers, while providing current buffering, do not provide current filtering. Also, existing current buffer amplifiers ordinarily do not provide near-perfect linearity at output.
FIG. 1 illustrates a conventional current buffer amplifier 100 that comprises unipolar transistors 102, 104, 112, 114, 116, and 118 (e.g., FET common gate connected transistors), differential amplifiers 106 and 110, a phase shift amplifier 108, and resistors 120 and 122. The differential amplifiers 106 and 110 are common mode feedback (CMFB) amplifiers used to suppress common-mode signals. The drains of the transistors 112 and 114 are connected to the inputs of the amplifiers 106 and 108 as shown in FIG. 1. The two input ports of the buffer amplifier are connected to the drains of the transistors 112 and 114 respectively. The positive input port is connected to a negative input port of the amplifier 108, the source of the transistor 102, and a negative input port of the amplifier 106. The negative input port is connected to a positive input port of the amplifier 108, the source of the transistor 106, and a positive input port of the amplifier 106. The input current iin is an output current from a first circuit (not shown) which is transferred to the second circuit (not shown) as output current iout by the current buffer amplifier 100. When both iin+ and iin− are applied, the CMFB amplifier 106 will amplify the iin input signal and the FET transistors 102, 104, 112, 114 will invert the signal (180° phase) and substrate the common signal. For differential signal CMFB would not operate. In this case, the input current I in will pass through. Thus, the input impedance gm1 is kept low and the output impedance gm2 is kept high.
Ideally, a current buffer amplifier is perfectly linear, with the output signal strength varying in direct proportion to the input signal strength. In a linear device, the output-to-input signal amplitude ratio is always the same, no matter what the strength of the input signal. A graph 200 in FIG. 2 illustrates an ideal current transfer gain as a function of frequency.
In reality, however, the type of ideal linearity illustrated in FIG. 2 is difficult to accomplish. Even if an amplifier exhibits linearity under normal conditions, it will become nonlinear if the input signal is too strong due to overdrive. The amplification curve bends toward a horizontal slope as the input-signal amplitude increases beyond a critical point, producing distortion in the output. In analog applications such as amplitude-modulation (AM), wireless transmission and hi-fi audio, linearity is important. Nonlinearity in these applications results in signal distortion because the fluctuation in gain affects the shape of an analog output waveform with respect to the analog input waveform. Accordingly, a linearity issue may arise in the current buffer amplifier illustrated in FIG. 1 when current is converted to voltage at the output.